In the field of lawn mowers, a need has been recognized for improved steering of lawn mowers. A particular need is apparent in the instance of hydraulically-driven lawn mowers. In hydraulically-driven lawn mowers, an engine (typically a gasoline-powered internal combustion engine) powers one or more hydraulic pumps, which drive two hydraulic motors which themselves drive wheels of a lawn mower. By controlling the flow of hydraulic fluid to the two hydraulic motors, the relative speed and rotation of the drive wheels may likewise be controlled. Such is shown in U.S. Pat. No. 3,876,021 to Baumgartner, incorporated by reference, a single engine drives two fluid pumps which drive two corresponding motors, each being coupled to a corresponding rear drive wheel. Control arms control the flow from the pumps.
A need has been recognized for a control system for such mowers which provides a "trim" feature, which allows the operator to finely adjust the relative speed of the drive wheels while the machine is moving. This may be especially needed when the operator has long stretches of straight-line operation; as may be understood, it is disadvantageous for an operator to have to continuously adjust the hand controls in order to keep the mower running in a straight line. By providing a trim feature, the operator can fine-tune the speed of the wheels to cause the mower to run in a substantially straight line. A trim feature may also be helpful when an operator is cutting on a hill; in such situations the mower may tend to turn downhill, and the trim feature can be used to compensate for such movement. A trim feature requiring no tools for adjustment would also be advantageous.
A need has also been recognized for a control lock feature which prevents actuation of controls either intentionally or unintentionally, unless the operator physically unlocks the controls.
Finally, a need has also been recognized for a latching feature which must be disengaged after the mower has been backed up.